The present invention generally relates to methods for producing degradable particulates, and methods related to the use of such degradable particulates in subterranean applications.
Degradable particulates comprise degradable materials (which are oftentimes degradable polymers) that are capable of undergoing an irreversible degradation when used in subterranean applications, e.g., in a well bore. As used herein, the terms “particulate” or “particulates” refer to a particle or particles that may have a physical shape of platelets, shavings, fibers, flakes, ribbons, rods, strips, spheroids, toroids, pellets, tablets, or any other suitable shape. The term “irreversible” as used herein means that the degradable material should degrade in situ (e.g., within a well bore) but should not recrystallize or reconsolidate in situ after degradation (e.g., in a well bore). The terms “degradation” or “degradable” refer to both of the two relatively extreme cases of hydrolytic degradation that the degradable material may undergo, e.g., heterogeneous (or bulk erosion) and homogeneous (or surface erosion), and any stage of degradation in between these two. This degradation can be a result of, inter alia, a chemical or thermal reaction, or a reaction induced by radiation. The terms “polymer” or “polymers” as used herein do not imply any particular degree of polymerization; for instance, oligomers are encompassed within this definition.
The degradability of a degradable polymer often depends, at least in part, on its backbone structure. For instance, the presence of hydrolyzable and/or oxidizable linkages in the backbone often yields a material that will degrade as described herein. The rates at which such polymers degrade are dependent on the type of repetitive unit, composition, sequence, length, molecular geometry, molecular weight, morphology (e.g., crystallinity, size of spherulites, and orientation), hydrophilicity, hydrophobicity, surface area, and additives. Also, the environment to which the polymer is subjected may affect how it degrades, e.g., temperature, presence of moisture, oxygen, microorganisms, enzymes, pH, and the like.
The physical properties of degradable polymers depend on several factors such as the composition of the repeat units, flexibility of the chain, presence of polar groups, molecular mass, degree of branching, crystallinity, orientation, etc. For example, short chain branches reduce the degree of crystallinity of polymers while long chain branches lower the melt viscosity and impart, inter alia, extensional viscosity with tension-stiffening behavior. The properties of the material utilized can be further tailored by blending, and copolymerizing it with another polymer, or by changing the macromolecular architecture (e.g., hyper-branched polymers, star-shaped, or dendrimers, etc.). The properties of any such suitable degradable polymers (e.g., hydrophobicity, hydrophilicity, rate of degradation, etc.) can be tailored by introducing select functional groups along the polymer chains.
Common methods that have been used to produce degradable particulates useful in subterranean applications (e.g., as acid precursors, fluid loss control particles, diverting agents, filter cake components, drilling fluid additives, cement additives, etc.) include, inter alia, emulsion methods and solution precipitation methods. To prepare degradable particulates using the emulsion method, typically a degradable polymeric material, such as poly(lactic acid), is dissolved in a halogenated solvent, e.g. methylene chloride, to form a polymeric solution and subsequently, water and a surfactant are then added to the polymeric solution at sufficient shear to form an emulsion. After formation of the emulsion, the solvent may then be removed from the emulsion by vacuum stripping or steam stripping, leaving behind essentially solvent-free particles of the polymer in the aqueous phase. The water is then removed and the particles may be collected by centrifugation, filtration, or spray-drying. Similarly, preparing degradable particulates with solution precipitation methods involves dissolution of a degradable polymer in a water miscible solvent to form a polymeric solution. Surfactants and/or water are then added to the polymeric solution with sufficient shear such that the solvent partitions from the polymeric solution, leaving behind essentially solvent-free particles of the polymer which may be collected by the same methods already discussed.
One problem associated with the current methods of producing degradable particulates is the necessity of surfactants and/or multiple solvents. Both the emulsion method and the solution precipitation method require the use of more than one solvent and/or surfactant. Furthermore, the halogenated solvents that may be used in these methods may pose health and environmental concerns. Thus, it may be beneficial and more cost-effective to have methods of producing degradable particulates that do not require the use of surfactants and/or multiple solvents, including halogenated solvents.